An embodiment of an OLED capable of emitting light having various colours is known from WO 2006/087654. In the known OLED an anode layer is provided on a suitable substrate, which is followed by a hole-injection layer followed by a layer of a light emissive material, having certain thickness along the substrate, above which a cathode layer is deposited.
OLEDs for signage applications typically require photolithography and wet-etching of layers to define the active area in the desired shape. Changing the topography of the OLED active area may lead to failure of those devices since the topography induces undesired thickness variations that make the device susceptible to catastrophic shorts. It is known that accuracy of thickness modulation of the light-emitting layer may affect the microcavity of the OLED system. Dimensioning the microcavity, with the effective wavelengths of the optical materials controls the outcoupling of the light emission. For example, a typical OLED stack may comprise a layer of a light-emitting polymer (LEP) cooperating with a hole injection layer and also possibly with an electron injection layer. In K. Neyts ‘Simulation of light emission from thin-film microcavities”, J. Opt. Soc. Am A Vol 15, No 4, April 1998, it is discussed how thicknesses of the optical stack may contribute to constructive or destructive interference, which will result in enhanced or reduced emission of light from the OLED.
This theory has resulted in a rigorous prescription of layer thicknesses to be used in the construction of the OLED stack. In WO2010117272 a proposition is made to provide a sub-layer of the OLED stack which is modulated in thickness, in order to optimize emission properties for certain wave lengths. A device according to the invention, comprising an OLED as is discussed with reference to the foregoing, may relate to a sensor, a display unit, a lighting arrangement or a signage unit.
However, tuning the thickness of the OLED stack may induce differences in electrical and optical behaviour, which is in practice challenging to control.